Gl Zhang

Helium decoration of impurities in metals

Abstract Equal numbers of He atoms and vacancies are liberated during the thermal annealing of helium-doped nickel samples at a temperature at which substitutional He atoms are released from the vacancies they occupy. We observed that the He atoms do not escape from the implanted region, but are retrapped in deeper traps. We have shown that He atoms can be quantitatively retrapped by In and Sb impurity atoms to form decorated defects, provided the He concentration exceeds a critical value of a few tenths of an atomic per cent. At lower concentrations bubble formation is probably the dominant retrapping mechanism. The decorated defects in nickel consist of several vacancies and He atoms, and are most likely occupied by 2 He atoms, arranged in a configuration with high symmetry such that the symmetry axis points along the 〈111〉 direction. This is the same as in copper, but different from aluminium where the orientation is along the 〈100〉 direction. Decorated defects are usually very stable against thermal annealing.

GROWTH AND PROPERTIES OF EPITAXIAL IRON OXIDE LAYERS

Epitaxial layers of iron oxides have been grown on a MgO(001) substrate by evaporating natural Fe or57Fe from Knudsen cells in the presence of a NO2 flow directed to the substrate. The resulting layers have been investigated in situ with LEED, RHEED, AES and XPS and ex situ with CEMS and ion beam analysis. For substrate temperatures between 200 and 400°C we observe RHEED oscillations during deposition, indicative of layer-by-layer growth. By adjusting the flux of NO2 at the surface, all stable and metastable cubic phases in the Fe-O system could be grown: FeO (wustite), Fe3O4 (magnetite), γ-Fe2O3 (maghemite) and solid solutions between the latter two phases. Rutherford backscattering spectra show a relatively high minimum yield in the channel directions.

A CEMS INVESTIGATION OF FE-57 IMPLANTED IN AL AND CU AT LOW-TEMPERATURE

A new type of CEM-spectrometer allows in situ measurements on metal foils implanted at low temperatures. It has been used to study defect association and clustering of57Fe in Al and Cu. For57FeAl, the substitutional fraction (fs) in samples implanted at 120 K is somewhat smaller than expected for a random impurity distribution but much larger than after room temperature implantation. For57FeCu,fs for samples implanted at 120 K is less than 0.5 of the value expected for a random distribution and it falls to zero after annealing at 600 K, where more extensive Fe clustering occurs. Vacancy trapping in stage III does not contribute significantly to the observed defect sites.

VACANCY TRAPPING AND HELIUM DECORATION IN SB ION-IMPLANTED TUNGSTEN STUDIED BY MOSSBAUER-SPECTROSCOPY

Mössbauer effect measurements have been performed using sources of119Sb implanted in W without and with post-implanted helium. Each of the sources was subjected to an isochronal annealing sequence in order to study vacancy trapping, helium decoration and recovery of damage. Four sites have been identified for Sb implanted in tungsten; one of these corresponds with substitutional Sb atoms, two others are assigned to Sb atoms associated with vacancies, while the last one can be either vacancy or impurity associated. The development of site occupation as a function of annealing temperature is in accordance with the one-interstitial model. Injection of 2·1016 He/cm2 leads to nucleation of helium bubbles. Helium atoms that are released from these bubbles at about 1300 K are retrapped by Sb atoms to form new bubbles.

MOSSBAUER STUDY ON IRON IN MARINE-SEDIMENTS AND MANGANESE NODULES

The chemical states of iron in a deep-sea sediment, a manganese nodule and a nearshore sediment are studied before and after chemical leaching. The Mössbauer spectra of the manganese nodule after chemical leaching clearly revealed the presence of a ferrous component not identified in previons studies. The ferrous component in the deep-sea sediment and the manganese nodule are shown to originate in terrigenous materials transported from lands through the atmosphere.

SIDEBANDS AND QUANTUM BEATS OF RECOILLESS GAMMA-RADIATION OF I-129

Abstract Sidebands intensities and quantum beats were measured at different vibration amplitudes of sources containing 129m Te, using the Mossbauer effect of 27.8 keV gamma rays emitted in its decay. The results can be understood in terms of a broad distribution of the amplitudes of the vibrating nuclei combined with a phase distribution with a standard deviation of 0.42 radians.

Europium-151 Mössbauer effect study of europium orthovanadite

Abstract151Eu Mössbauer spectroscopy on Eu VO3 in the temperature range of 9-293 K indicated that there is a large increase ofeVzzQg when temperature goes from 293K down to 9K.

VACANCY TRAPPING BY 5SP SERIES IMPURITIES IN ROLLED AND IMPLANTED NICKEL FOILS

Nickel foils containing substitutional119Sb,121Sb and125Sb impurities were deformed by rolling at room temperature and subsequently subjected to an isochronal annealing treatment. After each annealing step, a Mössbauer spectrum was measured. In the annealing temperature range from 350 to 800 K, in addition to the component for substitutional impurities, components ascribed to impurity atoms associated with one or more vacancies were observed in all cases. After annealing at 850 K and higher, only the substitutional component remains. Though very similar in general, some of the spectra for deformed119SbNi samples show clear differences from those obtained for implanted119SnNi samples. Additional experiments were performed on implanted sources of121Te and129Te in nickel. Here, substantial vacancy trapping is already present in undeformed and unannealed foils. Rolling of the129TeNi sources leads to a considerable increase of trapping after subsequent annealing at intermediate temperatures. For the insoluble Te impurities, a purely substitutional solution can not be recovered by annealing at higher temperatures. The evolution of vacancy trapping at the impurities that serve as probes and the dependence of the hyperfine field and the isomer shift of the impurities on the trapped vacancy configurations are discussed.

SIDEBAND AND QUANTUM-BEAT MEASUREMENTS WITH TE-129M AND CO-57 SOURCES

Sideband intensities and quantum beats were measured for recoillessly emitted gamma rays as a function of the vibration frequency of quartz crystals on which sources of129mTe and57Co were deposited. At most frequencies a low degree of coherence of the vibrations of the source nuclei was observed.